Electron discharge device with tunnel effect cathode and selectively scanned target



June 21, 1966 J. A. LODGE ETAL 3,257,581

ELECTRON DISCHARGE DEVICE WITH TUNNEL EFFECT CATHODE AND SELECTIVELY SGANNED TARGET Filed Oct. 15, 1962 FIG.1.

United States Patent 3,257,581 ELECTRON DISCHARGE DEVICE WITH TUN- NEL EFFECT CATHODE AND SELECTIVE- LY SCANNED TARGET James Alec Lodge, Sunbury-on-Thames, Middlesex, and Godfrey Newbold Hounsfield, South Muskham, near Newark, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Filed Oct. 15, 1962, Ser. No. 230,627 Claims priority, application Great Britain, Oct. 17, 1961, 37,098/ 61 Claims. (Cl. 315169) This invention relates to electron discharge devices and is especially, although not exclusively applicable to such devices for use for digital storage purposes.

In many digital storage devices a storage target is employed, on which are stored individual digits in an array, and in order to read out such a digit from the target it is required to cause electrons to impinge only on the particular area of the target on which the desired digit is stored. Access to the particular area is frequently difiicult and accordingly it is an object of the invention to provide an improved device in which electrons can be emitted selectively from different areas whereby this difficulty can be alleviated.

According to the invention there is provided an electron discharge device comprising a cathode including insulating material disposed between conducting material so that said cathode is capable of emitting electrons by the tunnel effect when suitable potentials are applied to said conducting material on opposite sides of said insulating material, means for receiving electrons from said cathode, and said insulating material being such that a potential difference can be applied with the aid of said conducting material selectively to different portions of said insulating material to cause electrons to be emitted selectively from elemental areas of said cathode. The phenomenon of the tunnel effect has been described in the Physical Review, 1958, volume 109, page 603. Tunnel emission of electrons into a vacuum has been found to occur when a layer of insulating material of suitable resistivity and thickness is disposed between two layers of conductive material which are held at suitable different potentials.

In order that the invention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawings.

FIGURE 1 is a plan view of a cathode of an electron discharge device according to one embodiment of the invention,

FIGURE 2 is a section through electrodes of the device, the cathode of which is illustrated in FIGURE 1, the envelope of the device being omitted in FIGURE 2,

FIGURE 3 is a diagrammatic representation of a cathode of an electron discharge device according to a further embodiment of the invention, and

FIGURE 4 shows an electron discharge device incorporating the electrodes shown in FIGURE 2.

In the drawing, the invention is shown, by way of example, as applied to an electron discharge device for digital storage, and as shown in FIGURES 1 and 2 the cathode of the device comrpises an insulating support 1, which may be of glass, having on its surface a plurality of spaced, parallel strips 2 of conducting material. By way of example the strips 2 may be of aluminium which is evaporated onto the glass support 1 using a suitable mask to mask areas of the support 1 on which the strips 2 are not required. At spaced positions along each strip 2 there are provided elements 3 of an insulating'material shown only in FIGURE 2. The insulating material is required where the strips 2 are crossed by other strips 4, but it may addi- Patented June 21, 1966 ice tionally be applied over the whole of the strips 2. The elements 3 may be in the form of an anodised layer of the strips 2 that is aluminum oxide, and may, by way of example, be about 70 A. in thickness. The insulating material may in some cases be semi-conducting material. A second set of spaced, parallel strips 4 of conducting material is then formed on the strips 2 and elements 3, the strips 4 being perpendicular to the strips 2 and disposed to lie over the elements 3 so that an array is formed of spaced elements each comprising a layer of insulating or semi-conducting material sandwiched between conducting layers. The strips 4 may be of aluminium or other suitable material and maye be formed by evaporation using a mask in the manner described for the strips 2. By way of example the thickness of the strips 4 may be about 50 A.

It is found that for each sandwich element, if the upper layer, namely strip 4, is biased sufficiently positively with respect to the lower layer, namely strip 2, then electrons from the lower layer will tunnel through the insulating layer, traverse the upper layer, and then have sufficient energy to overcome the potential barrier of the interface between said upper layer and the vacuum in the device so that said electrons are emitted into the vacuum. The amount of electron emission is very highly dependent upon the potential difference between the upper and lower layers of the sandwich element. Thus in order to operate the cathode described above a potential difference is applied between the upper and lower layers of a particular sandwich element from which it is required to emit electrons. Hence the particular two strips 2 and 4 of which parts form said upper and lower layers are pulsed, the strip 2 negatively and the strip 4 positively. Each pulse may, by way of example, be about 3 volts in amplitude. In view of the fact that the electron emission is such a steep function of the potential difference between the upper and lower layers, the emission from the sandwich element including the two pulsed strips 2 and 4 is very much greater than any emission from other sandwich elements including only one of the pulsed strips.

In order to increase the ratio of current at the wanted element to that at others, a steady reverse bias may be applied between the conducting strips 2 and 4 so that the ratio of the forward potential difference at the Wanted elements to that at unwanted elements is greater.

Spaced from the cathode is provided a target electrode as shown in FIGURE 2, said target electrode being maintained at a positive potential with respect to the cathode when the device 'is operating so that electrons emitted by the cathode are directed to said target electrode, whereby a particular area of said target electrode can be interrogated dependent upon the particular sandwich element of the cathode which is caused to emit electrons. The target electrode shown is of known construction and comprises a sheet 5 of insulating material which may, for example, be made of mica and be about 0.0015 inch thick. On the side of the sheet 5 remote from the cathode is a continuous conducting layer 6, and on the side of said sheet 5 facing the cathode is a mesh 7 of fine pitch, for example having 1,000 bars per linear inch. A further mesh 8 which acts as a collector is mounted between the cathode and target electrode and may conveniently be of about the same pitch as the mesh 7. The spacings between the cathode and the mesh 8, and mesh 8 and the target electrode may be a few millimetres. The cathode, target electrode and mesh 8 are mounted in an evacuated envelope 9 as shown in FIGURE 4, suitable lead-in wires being provided, passing through the envelope 9 to the strips 2 and 4, meshes 7 and 8 and conducting layer 6. The target electrode operates in the manner of that in the Radechon, an example of which is described in R.C.A. Review, June,

1955, pages 197 to 215, the cathode replacing the electron gun in the Radechon. Thus each sandwich element of the cathode corresponds to a section of the scanning raster of the electron beam produced by an electron gun in the Radechon. In order to write information onto the target electrode, the two strips 2 and 4 corresponding to a respective one of the sandwich elements are pulsed to cause electrons to be emitted by said sandwich element and to be incident on a corresponding section of the target electrode. During writing a potential difference is maintained between the mesh 7 and conducting layer 6, and the impinging electrons selectively charge up the sheet 5. During reading the potentials of mesh 7 and layer 6 are maintained equal so that all parts of the target with no information written thereon are at equilibrium potential whilst those parts which have information written on them are positive with respect to said equilibruim potential. When the electrons from the sandwich elements are incident upon the target where no information is written on the target, the number of secondary electrons leaving the target and reaching the collector mesh 8 is equal to the number of electrons incident upon the target. No displacement current then flows to the continuous electrode 6. If information in the form of a positive charge exists on the target in the area bombarded by electrons, then the potential is restored to equilibrium, that is, the potential of mesh 7, a displacement current flows in electrode 6 and the current of secondary electrons to mesh 8 is reduced. The output signal may be either the displacement current flowing to electrode 6 or the secondary electron current collected by mesh 8, which is always maintained at a potential more positive than that of mesh 7. In operation of the device the cathode, mesh 8 and target electrode are immersed in a magnetic field of magnitude approximately 0.1 weber/metre having its lines of force perpendicular to the planes of the cathode, mesh 8 and target electrode the magnetic field serving to focus electrons from the cathode to the target electrode. The mesh 8 may be maintained at 1,200 volts positive with respect to the mean potential of the cathode, and the mean potential of the mesh 7 and conducting layer 6 may be maintained 1,000 volts positive to the mean potential of the cathode.

In another embodiment of the invention, in order to reduce the number of connections to the device the cathode may effectively comprise a number of sections connected together in parallel. This is shown diagrammatically in FIGURE 3 in which the cathode is divided into four sections and each section (shown dotted) comprises four strips 2 and four strips 4. Corresponding strips of adjacent sections are connected together as shown so that the number of external connections is reduced from sixteen to eight. It will be appreciated, however, that when two strips are pulsed, four sandwich elements will emit electrons, and hence in order to suppress emission from all but one sandwich element, a set of grid electrodes is included between the cathode and mesh 8. The strips can be in the form of bands of fine pitch mesh or of single wires or ribbons. These further grid electrodes form a coarse matrix and in the case of four section cathode, may comprise a matrix, of two horizontal strips and two vertical strips with crossover points above the respective cathode sections, so that by applying suitable potentials to the strips electron projections to the target electrode may be suppressed from all but one cathode section. The number of strips in the coarse matrix will in any case depend upon the number of cathode sections which may obviously be greater than four.

Although the invention has been described with particular reference to the embodiments thereof shown in the drawings the invention is not restricted to such embodiments. For example the cathode may be employed in devices other than storage devices, such, for example, as a luminescent display tube, elemental areas of the screen of which are respectively associated with sandwich elements whereby the screen may be scanned by switching in cyclic order from one sandwich element to the next.

What we claim is:

1. An electron discharge device comprising a cathode incorporating insulating material of thickness and resistivity such that electrons can penetrate from one surface to another of said material by the tunnel effect, and conductors on said surfaces arranged such that a potential difference can be set up selectively across elements of said insulating material to cause electrons to penetrate a selected element and to be emitted from the area of said cathode occupied by said selected element, connections to said conductors for selectively setting up said potential difference, a target having elemental areas which can individually respond to the incidence of electrons, and means for causing electrons emitted from an area of said cathode occupied by a selected element to be incident on a corresponding elemental area of said target.

2. An electron discharge device comprising a cathode incorporating insulating material of thickness and resistivity such that electrons can penetrate from one surface to another of said material by the tunnel effect, a first set of spaced parallel conductors under said insulating material, and a second set of spaced parallel conductors, insulated from the first set and perpendicular thereto, over the insulating material, connections to said conductors for selectively setting up a potential difference across elements of said insulating material to cause electrons to penetrate a selected element and to be emitted from the area of said cathode occupied by said selected element, a target having elemental areas which can individually respond to the in cidence of electrons, and means for causing electrons emitted from an area of said cathode occupied by a selected element to be incident on a corresponding elemental area of said target.

3. A device according to claim 2 in which said target comprises a charge storage target.

4. An electron discharge device according to claim 2 in which said insulating material is confined to the regions where the conductors of said first and second sets overlie one another.

5. An electron discharge device comprising a cathode incorporating insulating material of thickness and resistivity such that electrons can penetrate from one surface to another of said material by the tunnel effect, a first set of spaced parallel conductors under said insulating material, and a second set of spaced parallel conductors, insulated from the first set and perpendicular thereto, over the insulating material, connections connecting said conductors to terminals such that each set of conductors has a plurality of groups and corresponding conductors of the same set but different groups are connected to respective common terminals so that potentials can be applied to two of said terminals, one for each set, to set up a potential difference selectively across a plurality of elements of said insulating material to cause electrons to penetrate the elements of the selected plurality and to be emitted from the areas of said cathode occupied by said selected plurality of elements, a target having elemental areas which can individually respond to the incidence of electrons, and means for causing electrons from only the area of said cathode occupied by one element of said selected group to be incident on a corresponding elemental area of said receiving surface.

6. An electron discharge device according to claim 5 in which said causing means includes a matrix of conductors disposed between said cathode and said receiving surface having a crossover point for each group of conductors.

7. An electron discharge device comprising a cathode of extended area incorporating insulating material of thickness and resistivity such that electrons can penetrate from one surface to another of said material by the tunnel effect, and conductors on said surfaces arranged such that a potential difference can be set up selectively across elements of said insulating material to cause electrons to penetrate a selected element and to be emitted from the area of said cathode occupied by said selected element, connections to said conductors for selectively setting up said potential difference, a target incorporating a sheet of substantially insulating material substantially parallel to said cathode, a conducting mesh on the surface of said sheet facing said cathode, a conducting layer on the surface of said sheet remote from said cathode, connections for applying potentials to said mesh and layer, and means for causing electrons emitted from an area of said cathode occupied by a selected element to be incident on a corresponding element-al area of said target electrode.

8. An electron discharge device according to claim 7 incorporating a mesh collecting electrode disposed between said cathode and said target for collecting secondary electrons emitted by said target.

9. A circuit arrangement embodying an electron discharge device comprising a cathode incorporating insulating material of thickness and resistivity such that electrons can penetrate from one surface to another of said material by the tunnel eifect, and conductors on said surfaces arranged such that a potential difference can be set up selectively across elements of said insulating material to cause electrons to penetrate a selected element and to be emitted from the area of said cathode occupied by said selected element, connections to said conductors and circuit means applying potentials to said connections to set up said selective potential difference and to suppress the emission of electrons from areas of said cathode occupied by elements which are not selected, a target having elemental areas which can individually respond to the incidence of electrons, and means for causing electrons emitted from an area of said cathode occupied by a selected element to be incident on a corresponding elemental area of said receiving surface.

10. A circuit arrangement embodying an electron discharge device comprising a cathode incorporating insulating material of thickness and resistivity such that electrons can penetrate from one surface to another of said material by the tunnel efiect, a first set of spaced parallel 6 conductors under said insulating material, and a second set of spaced parallel conductors, insulated from the first set and perpendicular thereto, over the insulating material, connections connecting said conductors to terminals such that each set of conductors has a plurality of groups and corresponding conductors of the same set but different groups are connected to respective common terminals, circuit means applying potentials to tWo of said terminals, one for each set, setting up a potential difference selectively across a plurality of elements of said insulating material to cause electrons to penetrate the elements of the selected plurality and to be emitted from the areas of said cathode occupied by said selected plurality of elements, a target having elemental areas which can individually respond to the incidence of electrons, a matrix of conductors disposed between said cathode and said receiving surface having a cross over point for each group of said first and second sets of conductors, circuit means applying suppresing potentials to all of said matrix of conductors except those whose crossing point are most adjacent one element of said selected plurality, and means for causing electrons from the area of said cathode occupied by said one element to be incident on a corresponding elemental area of said target.

References Cited by the Examiner UNITED STATES PATENTS 2,201,066 5/1940 Toulon 178-7.5 2,474,338 6/ 1949 Toulon 17857.5 2,796,549 6/1957 Fiske 315-1 2,926,286 2/1960 Skellett 313346 X 3,056,073 9/1962 Mead.

3,107,315 10/1963 Wolfe et a1. 315--169 FOREIGN PATENTS 1,143,341 11/1958 France.

GEORGE N. WESTBY, Primary Examiner.

ROBERT SEGAL, Assistant Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A CATHODE INCORPORATING INSULATING MATERIAL OF THICKNESS AND RESISTIVITY SUCH THAT ELECTRONS CAN PENETRATE FROM ONE SURFACE TO ANOTHER OF SAID MATERIAL BY THE TUNNEL EFFECT, AND CONDUCTORS ON SAID SURFACES ARRANGED SUCH THAT A POTENTIAL DIFFERENCE CAN BE SET UP SELECTIVELY ACROSS ELEMENTS OF SAID INSULATING MATERIAL TO CAUSE ELECTRONS TO PENETRATE A SELECTED ELEMENT AND TO BE EMITTED FROM THE AREA OF SAID CATHODE OCCUPIED BY SAID SELECTED ELEMENT, CONNECTIONS TO SAID CONDUCTORS FOR SELECTIVELY SETTING UP SAID POTENTIAL DIFFERENCE, A TARGET HAVING ELEMENTAL AREAS WHICH CAN INDIVIDUALLY RESPOND TO THE INCIDENCE OF ELECTRONS, AND MEANS FOR CAUSING ELECTRONS EMITTED FROM AN AREA OF SAID CATHODE OCCUPIED BY A SELECTED ELEMENT TO BE INCIDENT ON A CORRESPONDING ELEMENTAL AREA OF SAID TARGET. 